Method of performing uplink synchronization in wireless communication system

ABSTRACT

A method of performing uplink synchronization in a wireless communication system includes transmitting a random access preamble which is randomly selected from a set of random access preambles, receiving a random access response, the random access response comprising a random access preamble identifier corresponding to the random access preamble and a time alignment value for uplink synchronization, starting a time alignment timer after applying the time alignment value, starting a contention resolution timer after receiving the random access response, wherein contention resolution is not successful when the contention resolution timer is expired, and stopping the time alignment timer when the contention resolution timer is expired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisional application Ser. No. 60/944,785 filed on June 18 and Korean Patent Application No. 10-2008-0023809 filed on Mar. 14, 2008, which are incorporated by reference in its entirety herein.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications, and more particularly, to a method of performing uplink synchronization in a wireless communication system.

2. Related Art

Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.

In general, there are one or more cells within the coverage of a base station (BS). One cell may include a plurality of UEs. A UE is generally subjected to a random access procedure to access a network. The random access procedure is performed by the UE for the following purposes: (1) initial access; (2) handover; (3) scheduling request; and (4) timing synchronization. However, this is for exemplary purposes only, and thus the content or number of purposes for performing the random access procedure may vary depending on systems.

The random access procedure can be classified into a contention based random access procedure and a non-contention based random access procedure. Major difference between the two random access procedures lies in whether a random access preamble is dedicatedly assigned to one UE. In the non-contention based access procedure, since a UE uses only the random access preamble dedicatedly assigned to the UE, contention (or collision) with another UE does not occur. The contention occurs when two or more UEs attempt the random access procedure by using the same random access preamble through the same resource. In the contention based random access procedure, there is a possibility of contention since a random access preamble used by the UEs is randomly selected.

In an orthogonal frequency division multiplexing (OFDM)-based wireless communication system, timing synchronization between a UE and a BS is important so as to minimize interference between users. The random access procedure is performed for uplink synchronization. While the random access procedure is performed, the UE is time-synchronized according to a time alignment value transmitted from the BS. When uplink synchronization is achieved, the UE runs a time alignment timer. If the time alignment timer is running, it is regarded that the UE and the BS are uplink-synchronized with each other. If the time alignment timer is expired or is not running, it is regarded that the UE and the BS are not synchronized with each other. In this case, uplink transmission cannot be achieved except for transmission of the random access preamble.

A random access failure may occur in the contention based random access procedure since there is always a possibility of contention. Therefore, a method is needed for performing uplink synchronization caused by the random access failure.

SUMMARY

The present invention provides a method of performing uplink synchronization while a contention based random access procedure is performed in a wireless communication system.

The present invention also provides a method of avoiding interference to other user equipments due to incorrect uplink synchronization in a wireless communication system.

In an aspect, a method of performing uplink synchronization in a wireless communication system is provided. The method includes transmitting a random access preamble which is randomly selected from a set of random access preambles, receiving a random access response, the random access response comprising a random access preamble identifier corresponding to the random access preamble and a time alignment value for uplink synchronization, starting a time alignment timer after applying the time alignment value, starting a contention resolution timer after receiving the random access response, wherein contention resolution is not successful when the contention resolution timer is expired, and stopping the time alignment timer when the contention resolution timer is expired.

The method can further include transmitting a scheduled message, the scheduled message comprising a unique identifier and stopping the contention resolution timer when receiving a contention resolution message, the contention resolution message comprising an identifier corresponding to the unique identifier. The contention resolution timer may be started when transmitting the scheduled message.

In another aspect, a method of performing uplink synchronization in a wireless communication system is provided. The method includes transmitting a random access preamble which is randomly selected from a set of random access preambles, receiving a random access response, the random access response comprising a random access preamble identifier corresponding to the random access preamble, starting a time alignment timer after receiving a random access response and stopping the time alignment timer when contention resolution is not successful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 is a diagram showing functional split between an evolved universal terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC).

FIG. 3 is a block diagram showing constitutional elements of a user equipment (UE).

FIG. 4 is a diagram showing a radio protocol architecture for a user plane.

FIG. 5 is a diagram showing a radio protocol architecture for a control plane.

FIG. 6 is a flow diagram showing a random access procedure.

FIG. 7 is a flow diagram showing a method of performing uplink synchronization according to an embodiment of the present invention.

FIG. 8 is a flow diagram showing a method of performing uplink synchronization according to another embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a structure of a wireless communication system. The wireless communication system may have a network structure of an evolved-universal mobile telecommunications system (E-UMTS). The E-UMTS may be referred to as a long-term evolution (LTE) system. The wireless communication system can be widely deployed to provide a variety of communication services, such as voices, packet data, etc.

Referring to FIG. 1, an evolved-UMTS terrestrial radio access network (E-UTRAN) includes at least one base station (BS) 20 which provides a control plane and a user plane.

A user equipment (UE) 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc. There are one or more cells within the coverage of the BS 20. Interfaces for transmitting user traffic or control traffic may be used between the BSs 20. Hereinafter, downlink is defined as a communication link from the BS 20 to the UE 10, and uplink is defined as a communication link from the UE 10 to the BS 20.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20 are also connected by means of an S1 interface to an evolved packet core (EPC), more specifically, to a mobility management entity (MME)/serving gateway (S-GW) 30. The S1 interface supports a many-to-may relation between the BS 20 and the MME/S-GW 30.

FIG. 2 is a diagram showing functional split between the E-UTRAN and the EPC.

Referring to FIG. 2, slashed boxes indicate radio protocol layers and white boxes indicate functional entities of the control plane.

The BS performs the following functions: (1) functions for radio resource management (RRM) such as radio bearer control, radio admission control, connection mobility control, and dynamic allocation of resources to the UE; (2) Internet protocol (IP) header compression and encryption of user data streams; (3) routing of user plane data to the S-GW; (4) scheduling and transmission of paging messages; (5) scheduling and transmission of broadcast information; and (6) measurement and measurement reporting configuration for mobility and scheduling.

The MME performs the following functions: (1) distribution of paging messages to the BSs; (2) security control; (3) idle state mobility control; (4) system architecture evolution (SAE) bearer control; and (5) ciphering and integrity protection of non-access stratum (NAS) signaling.

The S-GW performs the following functions: (1) termination of a user plane packet for paging; and (2) user plane switching for the support of UE mobility.

FIG. 3 is a block diagram showing constitutional elements of the UE. A UE 50 includes a processor 51, a memory 52, a radio frequency (RF) unit 53, a display unit 54, and a user interface unit 55. Layers of a radio interface protocol are implemented in the processor 51. The processor 51 provides the control plane and the user plane. The function of each layer can be implemented in the processor 51. The memory 52 is coupled to the processor 51 and stores an operating system, applications, and general files. The display unit 54 displays a variety of information of the UE 50 and may use a well-known element such as a liquid crystal display (LCD), an organic light emitting diode (OLED), etc. The user interface unit 55 can be configured with a combination of well-known user interfaces such as a keypad, a touch screen, etc. The RF unit 53 is coupled to the processor 51 and transmits and/or receives radio signals.

Layers of a radio interface protocol between the UE and the network can be classified into L1 layer (a first layer), L2 layer (a second layer), and L3 layer (a third layer) based on the lower three layers of the open system interconnection (OSI) model that is well-known in a communication system. A physical layer, or simply a PHY layer, belongs to the first layer and provides an information transfer service through a physical channel. A radio resource control (RRC) layer belongs to the third layer and serves to control radio resources between the UE and the network. The UE and the network exchange RRC messages via the RRC layer.

FIG. 4 is a diagram showing a radio protocol architecture for the user plane. FIG. 5 is a diagram showing a radio protocol architecture for the control plane. They illustrate an architecture of a radio interface protocol between the UE and the E-UTRAN. The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission.

Referring to FIGS. 4 and 5, a PHY layer belongs to the first layer and provides an upper layer with an information transfer service through a physical channel. The PHY layer is coupled with a medium access control (MAC) layer, i.e., an upper layer of the PHY layer, through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers (i.e., a PHY layer of a transmitter and a PHY layer of a receiver), data are transferred through the physical channel. The PHY layer can be modulated by orthogonal frequency division multiplexing (OFDM). Time and/or frequency can be utilized as radio resources.

The MAC layer belongs to the second layer and provides services to a radio link control (RLC) layer, i.e., an upper layer of the MAC layer, through a logical channel. The RLC layer in the second layer supports reliable data transfer. There are three operating modes in the RLC layer, that is, a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM) according to a data transfer method. An AM RLC provides bidirectional data transmission services and supports retransmission when the transfer of a RLC protocol data unit (PDU) fails.

A packet data convergence protocol (PDCP) layer belongs to the second layer and performs a header compression function. When transmitting an IP packet such as an IPv4 packet or an IPv6 packet, a header of the IP packet may contain relatively large and unnecessary control information. The PDCP layer reduces a header size of the IP packet so as to efficiently transmit the IP packet.

A radio resource control (RRC) layer belongs to the third layer and is defined only in the control plane. The RRC layer serves to control the logical channel, the transport channel, and the physical channel in association with configuration, reconfiguration and release of radio bearers (RBs). A RB is a service provided by the second layer for data transmission between the UE and the E-UTRAN. When an RRC connection is established between an RRC layer of the UE and an RRC layer of the network, it is called that the UE is in an RRC connected mode. When the RRC connection is not established yet, it is called that the UE is in an RRC idle mode.

A non-access stratum (NAS) layer belongs to an upper layer of the RRC layer and serves to perform session management and mobility management.

A downlink transport channel is a channel through which data is transmitted from the network to the UE. Examples of the downlink transport channel include a broadcast channel (BCH) for transmitting system information and a downlink-shared channel (DL-SCH) for transmitting user traffic or control messages. User traffic of downlink multicast (or broadcast) services or control messages can be transmitted on the DL-SCH or a downlink multicast channel (MCH). An uplink transport channel is a channel through which data is transmitted from the UE to the network. Examples of the uplink transport channel include a random access channel (RACH) for transmitting initial control messages and an uplink-shared channel (UL-SCH) for transmitting user traffic or control messages.

A downlink physical channel is mapped to the downlink transport channel. Examples of the downlink physical channel include a physical broadcast channel (PBCH) for transmitting information of the BCH, a physical multicast channel (PMCH) for transmitting information of the MCH, a physical downlink shared channel (PDSCH) for transmitting information of the PCH and the DL-SCH, and a physical downlink control channel (PDCCH) for transmitting control information such as downlink or uplink scheduling grant, which are provided from the first layer and the second layer. The PDCCH is also referred to as a downlink L1/L2 control channel. An uplink physical channel is mapped to the uplink transport channel. Examples of the uplink physical channel include a physical uplink shared channel (PUSCH) for transmitting information of the UL-SCH, a physical random access channel (PRACH) for transmitting information of the RACH, and a physical uplink control channel (PUCCH) for transmitting control information such as hybrid automatic repeat request (HARQ) acknowledgement (ACK)/non-acknowledgement (NACK) signals, a scheduling request signal, and a channel quality indicator (CQI), which are provided from the first layer and the second layer.

Now, the random access procedure will be described. A UE performs the random access procedure in the process of performing the following operations, such as, (1) initial access, (2) handover, (3) transmission of downlink data to a non-synchronized UE, (4) transmission of uplink data by the non-synchronized UE, and (5) restoration of radio link failure.

FIG. 6 is a flow diagram showing the random access procedure.

Referring to FIG. 6, in step S110, a UE transmits a random access preamble to a BS through a selected random access resource by using system information received from the BS. The system information includes information on a set of available random access preambles. The random access preamble transmitted by the UE is selected from the set of random access preambles.

In step S120, the BS transmits a random access response through a DL-SCH. The random access response includes a time alignment value for uplink synchronization of the UE, uplink radio resource allocation information, an index of the random access preamble received to identify the UE performing the random access procedure, and a temporary identifier (e.g., temporary cell-radio network temporary identity (C-RNTI)) of the UE.

In step S130, the UE applies the time alignment value, and transmits a scheduled message including a unique identifier of the UE to the BS by using the uplink radio resource allocation information. The unique identifier of the UE may be the C-RNTI, a SAE temporary mobile station identifier (S-TMSI), or an upper-layer identifier. The unique identifier is also referred to as a contention resolution identifier since it is used for contention resolution.

In step S140, after receiving the scheduled message, the BS transmits to the UE a contention resolution message including the unique identifier of the UE.

Due to the limited number of available random access preambles, contention occurs in the random access procedure. Since it is not possible to assign unique random access preambles to all UEs located in a cell, the UEs select one random access preamble from the set of the available random access preambles and then transmit the selected random access preamble. Accordingly, two or more UEs can select and transmit the same random access preamble through the same random access resource. This is a case where contention occurs. Upon receiving the random access preamble, the BS transmits the random access response for the random access preamble in a state where the BS does not know the occurrence of contention. However, since contention has occurred, two or more UEs receive the same random access response and thus transmit scheduled messages according to information included in the random access response. That is, the two or more UEs transmit different scheduled messages according to the uplink radio resource allocation information included in the random access response. In this case, the BS may fail to receive all of the scheduled messages, or may successfully receive only a scheduled message of a specific UE according to the location or transmit (Tx) power of the UEs. If the BS successfully receives the scheduled message, the BS transmits the contention resolution message by using the unique identifier of the UE, wherein the unique identifier is included in the scheduled message. The UE can know that the contention resolution is successful when the unique identifier of the UE is received. The contention resolution allows the UE to know whether the contention is successful or not in the contention based random access procedure.

A contention resolution timer is used for the contention resolution. The contention resolution timer starts after the random access response is received. The contention resolution timer may start when the UE transmits the scheduled message. When the contention resolution timer is expired, it is determined that the contention resolution is unsuccessful, and thus a new random access procedure is performed. When the UE receives the contention resolution message including the unique identifier of the UE, the contention resolution timer stops, and the UE determines that the contention resolution is successful. If the UE already has a unique cell identifier (e.g., C-RNTI) before the random access procedure is performed, the UE transmits the scheduled message including the cell identifier of the UE and then starts the contention resolution timer. If the UE receives a PDCCH, which is addressed by the cell identifier of the UE, before the contention resolution timer is expired, the UE determines that the contention is successful and then finishes the random access procedure without errors. If the UE does not have the C-RNTI, the upper-layer identifier may be used as the unique identifier. After transmitting the scheduled message including the upper-layer identifier, the UE starts the contention resolution timer. If the contention resolution message including the upper-layer identifier of the UE is received on the DL-SCH before the contention resolution timer is expired, the UE determines that the random access procedure is successful. The contention resolution message is received by using the PDCCH addressed by the temporary C-RNTI. Otherwise, if the aforementioned contention resolution is not received on the DL-SCH until the contention resolution timer is expired, the UE determines that the contention is unsuccessful.

Now, time alignment for uplink synchronization will be described. In an OFDM-based system, timing synchronization between a UE and a BS is important so as to minimize interference between users.

The random access procedure is one of uplink time synchronization methods. That is, the BS measures the time alignment value through the random access preamble transmitted by the UE, and provides the time alignment value to the UE through the random access response. Upon receiving the random access response, the UE applies the time alignment value and starts the time alignment timer. Time synchronization between the UE and the BS is maintained while the time alignment timer is running. If the time alignment timer is expired or is not running, it is regarded that the time synchronization between the UE and the BS is not maintained. If the time alignment timer is expired and is not running, the UE can transmit only the random access preamble, and any other uplink transmission cannot be achieved.

When contention occurs in the process of performing the random access procedure, the UE may apply an incorrect time alignment value. If the UE is not time-synchronized with the BS before the UE transmits the random access preamble, the random access preamble may be mistakenly transmitted in uplink because of the time alignment timer currently running.

First, in a state whether uplink synchronization is not achieved between the UE and the BS, the UE transmits to the BS the random access preamble which is randomly selected, and receives the random access response for the random access preamble. Even if contention occurs, the UE can receive the random access response. In this case, since the UE cannot know the occurrence of contention, the UE applies to the UE itself the time alignment value included in the received random access response, and starts the time alignment timer. Subsequently, the UE transmits to the BS the scheduled message including the unique identifier of the UE and starts the contention resolution timer. If the UE does not receive the contention resolution message addressed by the unique identifier of the UE until the contention resolution timer is expired, the UE retries the random access procedure. However, since the time alignment timer is continuously running, the UE may transmit uplink data when the BS transmits downlink data. This is because the UE determines that uplink synchronization is achieved by the time alignment timer currently running, even in a case where uplink synchronization is not achieved. The incorrect uplink synchronization may result in interference to other users during transmission.

FIG. 7 is a flow diagram showing a method of performing uplink synchronization according to an embodiment of the present invention. At first, a UE is in a state where a time alignment timer is expired or is not running. This is a case where a contention based random access procedure starts when the UE attempts initial network entry or when a cell is searched for again due to a radio link failure.

Referring to FIG. 7, in step S210, the UE transmits to a BS a random access preamble which is randomly selected. In step S220, in response to the random access preamble, the BS transmits a random access response to the UE. The random access response includes uplink radio resource allocation information, a random access preamble identifier, a time alignment value, and a temporary C-RNTI. In step S230, the UE applies the time alignment value included in the random access response, and starts the time alignment timer.

In step S240, through the uplink radio resource allocation information included in the random access response, the UE transmits to the BS a scheduled message including a unique identifier of the UE. In step S250. the UE transmits the scheduled message and then starts a contention resolution timer.

In step S260, if the UE does not receive a contention resolution message including the unique identifier of the UE until the contention resolution timer is expired, the UE stops the time alignment timer. In this case, the UE may determine that the contention is unsuccessful, and thus retry the random access procedure.

If the random access failure occurs, the UE stops the time alignment timer which has been previously running. This is because the previously received time alignment value can be for another UE. Therefore, since the time alignment timer is stopped, uplink transmission is prevented from using the incorrect time alignment value.

FIG. 8 is a flow diagram showing a method of performing uplink synchronization according to another embodiment of the present invention.

Referring to FIG. 8, in step S310, a UE transmits to a BS a random access preamble which is randomly selected. In step S320, in response to the random access preamble, the BS transmits a random access response to the UE. The random access response includes a time alignment value and temporary C-RNTI. In step S330, the UE applies the time alignment value and starts a time alignment timer. In step S340, the UE transmits to the BS a scheduled message including a unique identifier of the UE through uplink radio resource allocation information included in the random access response. In step S350, after transmitting the scheduled message, the UE starts a contention resolution timer.

In step S360, before the contention resolution timer is expired, the UE receives a contention resolution message indicated by PDCCH addressed by the temporary C-RNTI. The contention resolution message includes a contention resolution identifier. In step S370, the UE determines whether the contention is successful through the contention resolution identifier. For example, the contention resolution identifier can be compared with the identifier included in the scheduled message. If the two identifiers are not identical, it is determined that the contention is unsuccessful.

In step S380, if it is determined that the contention is unsuccessful, the UE stops the time alignment timer. In this case, the UE may retry the random access procedure or may report to an upper layer that the contention is unsuccessful.

If the contention is unsuccessful in the random access procedure, the UE stops the time alignment timer currently running. Accordingly, a problem can be solved in which the time alignment timer continuously runs even when uplink synchronization is not achieved.

Incorrect uplink synchronization can be avoided even if contention resolution is unsuccessful. Therefore, interference to other user equipments can be mitigated and service delay due to incorrect uplink synchronization can be prevented.

The steps of a method described in connection with the embodiments disclosed herein may be implemented by hardware, software or a combination thereof. The hardware may be implemented by an application specific integrated circuit (ASIC) that is designed to perform the above function, a digital signal processing (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microprocessor, the other electronic unit, or a combination thereof. A module for performing the above function may implement the software. The software may be stored in a memory unit and executed by a processor. The memory unit or the processor may employ a variety of means that is well known to those skilled in the art.

As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims. Therefore, all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are intended to be embraced by the appended claims. 

What is claimed:
 1. A method of performing uplink synchronization in a wireless communication system, the method comprising: transmitting, by a user equipment (UE), a random access preamble to a network; receiving, by the UE, a random access response comprising a timing alignment command from the network, wherein the random access response is identified by a Random Access Radio Network Temporary Identifier (RA-RNTI); starting, by the UE, a time alignment timer after receiving the timing alignment command; and stopping, by the UE, the time alignment timer when contention resolution between the UE and the network is considered not successful.
 2. The method of claim 1, wherein the random access response further comprises a temporary UE identifier.
 3. The method of claim 1, wherein the random access preamble is selected by the UE from a set of random access preambles.
 4. The method of claim 1, further comprising: starting, by the UE, a contention resolution timer after receiving the random access response.
 5. The method of claim 4, wherein the contention resolution is considered not successful if the contention resolution timer is expired.
 6. A user equipment (UE) of performing uplink synchronization in a wireless communication system, the UE comprising: a transceiver; and a processor coupled with the transceiver and configured to transmit a random access preamble to a network; receive a random access response comprising a timing alignment command from the network, wherein the random access response is identified by a Random Access Radio Network Temporary Identifier (RA-RNTI); start a time alignment timer after receiving the timing alignment command; and stop the time alignment timer when contention resolution between the UE and the network is considered not successful.
 7. The UE of claim 6, wherein the random access response further comprises a temporary UE identifier.
 8. The UE of claim 6, wherein the random access preamble is selected by the UE from a set of random access preambles.
 9. The UE of claim 6, wherein the processor is further configured to start a contention resolution timer after receiving the random access response.
 10. The UE of claim 9, wherein the contention resolution is considered not successful if the contention resolution timer is expired. 